Adense Epic Dungeon is a large dungeon crawl that offers a variety of environments, which keeps the action from getting tedious and offers a welcome break from the typically same-y environments in Oblivion's caves and ruins.

The first challenge will be to get past the initial area. There are two main difficulties here, and depending on how you feel about these things, they either reflect bad cell design or Oblivion's engine limitations.

In addition to the glowing gate, which has a lot of particles for the engine to calculate and draw, the area is large and densely populated with many objects. Even if you turn your camera away from the gate and put some distance from it, your may still encounter an unacceptable drop in your frame rate.

Size isn't the issue here so much as is the number of separate items that the game engine has to process. Even items that are entirely concealed behind another object must be calculated by the engine to determine whether it needs to be rendered or not.

Fortunately, there aren't many adversaries here, otherwise combat could be unplayably sluggish to the point where Oblivion can freeze your system.

In the initial area, the design clearly trades FPS for "interestingness", although it certainly could have been done better -- There are a few areas where the author could easily have made a cell transition, thus reducing the overall load on the engine.

If you can't be bothered to exit your game and choose a much smaller screen resolution, you can get through low-FPS areas or encounters by using the console command "TLB" (ToggleLiteBrite). This appears to basically turn off the calculation of lighting, and can free up your FPS by a considerable amount even on a high screen resolution. Some areas will be covered with a plane of colour, concealing things; there is also the possibility that the visual distortion FX from summoning a creature can crash the game. But otherwise, temporarily turning this on can give you the FPS boost to fight a large battle with appropriate responsiveness to your mouse clicks and keystrokes.

The other issue in the initial areas is an over-use of rocky terrain. While this gives an interesting look and navigation, Oblivion handles rocks and slopes very poorly. There is typically no guarantee from looking at terrain as to whether you will be able to run up, or even jump up. Sometimes, your character will sink up to their ankles in what appeared to be solid rock. Other times, your character will not be able to jump when they could have in a similar situation. When running about in the wilderness, you can usually simply walk around, but here, you may very well get stuck.
If you are frustrated, you can use the console command TCL (Toggle Collision -- make sure you don't have any object or creature selected in the console, but clicking on blank space and checking to make sure there is no label in the top middle of your screen while in console mode) to ignore barriers and gravity and walk to where you need to be.
After the initial area, these issues typically do not happen.

TIP: There is one event in the early stage where you will trigger a collapse and a key will be behind the rockfall area. It is possible that the arrangement of the debris will be such that you cannot get around or over it, in which case you will probably need TCL to get past.

It bears repeating that the issues of lag and rocky terrain are more or less (depending on your computer) confined to the initial big area and once you're past that, the rest of the worldspaces in the mod should be fine.

The distribution of enemies is varied and just because you could get past a few of the grunts doesn't mean you won't be overwhelmed later in a place where enemies are clumped together.
TIP: The Barracks also features an optional boss you may accidentally trigger. There is no real benefit to defeating him and he is suddenly of a very much higher difficulty than the rest of the encounters. There is the possibility of luring him into a wide-area trap, but it is probably easier to simply not trigger him and move on.

The loot along the way is typically useful for an extended trip, with some potions, soul gems, and directly useful creature ingredients with such effects as Damage Health or Invisibility. (More repair hammers might have been nice, though).

The frequent deep darkness is novel and nicely done as it is generally not necessary to use lighting all the time (although on your first run you can't possibly know this). Sometimes mod authors will deliberately conceal deadly traps in the darkness, but this just encourages players to dispel the darkness all the time, which in turn defeats the purpose of having darkness-based ambiance.

The backstory to why the place looks so ruined has merit, but doesn't really fit with the actual environment.
Adense Epic Dungeon advertises, "It is said that Malacath has a hidden sanctuary where he tests the bravery of warriors who think themselves worthy of serving him. That sanctuary is said to be a place of challenges worthy of any warrior." We can glimpse the pre-ruined architecture, but it has too much of an Ayleid influence to really look like a warrior's testing ground that Malacath might have built. As for the later areas, it becomes hard to tell which areas are meant to be part of Malacath's testing ground.Vaermina is invading and is supposedly the reason the initial locations look like an atomic bomb ground zero, but amphibious creatures with glow-in-the-dark bits doesn't feel like the sort of minions Vaermina might use.
Possibly depending on the mods you have, Vaermina looks like a pretty hot and leggy version of Eyja, which in turn is nothing like her statues or other depictions.

Still, if you don't let such details of how "Lore-friendly" the backstory is bother you, it works well as a dungeon crawl and there are sights worth seeing along the way. The mod offers interesting sidetrips and encounters, such as the possibility of gaining companions. Deserving of special mention is the dialogue for the first (live) orc adventurer you meet, Grom gro-Karrack.